1. Field of the Invention
The present invention relates to a control apparatus for driving a moving body using a vibration actuator such as a so-called ultrasonic motor.
2. Related Background Art
In recent years, a vibrator actuator, e.g., a so-called ultrasonic motor, which utilizes an electro-mechanical energy conversion element such as a piezo-electric element, an electro-strictive element, or the like, has been put into practical applications. The vibration actuator comprises a vibrating body which is vibrated by an AC power supply, and a moving body which is moved by vibration of the vibrating body while being urged against the surface of the vibrating body. In speed control of the vibration actuator (i.e., speed control of the moving body), a change in position of the moving body is detected by a detector such as an encoder mechanically connected to the moving body, and the output from the encoder is calculated by a microcomputer to detect the speed of the moving body. Thus, the vibration of the vibrating body is controlled in accordance with the detected speed.
In the conventional speed control method, a change in instantaneous position of the moving body is detected by the encoder. However, the moving body is not only moved but also vibrated since it is in contact with the vibrating body. For this reason, the encoder for detecting the moving amount of the moving body actually detects a superposed motion of the movement and the vibration of the moving body, resulting in poor reliability of the encoder output. In the conventional speed control method, when the speed of the moving body is high, the influence of the vibration of the vibrating body is relatively decreased, and the control performance is not substantially impaired. However, when the moving speed of the moving body is very low, the influence of the vibration of the moving body seriously affects the encoder output, and the precision and reliability of the encoder output are considerably impaired. Therefore, the conventional speed control method described above is not suitable for speed control of a low-speed vibration actuator or of a vibration actuator at low speed.
Even when position control is performed using the encoder, since the vibration of the vibration frequency component of the vibration actuator is superposed on the output from a position detection sensor, position data, which has a given variation width having an actual actuator position as the center, is detected, resulting in unstable position control.